In tube-type heat exchangers and other vessels used for industrial processes, the tubes extend between tubesheets and have their ends secured to the tubesheets. One fluid fills the cavity between the tubesheets and surrounds the outsides of the tubes, while another flows through the tubes, so that heat is transferred through the tube walls from the hotter fluid to the cooler fluid. Various procedures and equipment have been developed for fastening the tubes to the tubesheets.
In the typical heat exchanger, each tube extends through a separate hole in the tubesheet and has its end edge flush with a front face of the tubesheet. Here, the two are welded together so that the weld exists along the end edge of the tube. Since the weld is at the front face of the tubesheet, a small annular crevice exists between the outer surface of the tube wall and the surrounding surface of the tubesheet hole into which the tube fits. In some applications, particularly where the fluid that circulates amongst the tubes is a liquid, the fluid enters the crevices surrounding the tubes. Where the liquid is water, chloride crevice stress corrosion may occur in the portion of the two components forming the crevice, particularly in the tubesheet hole and tube wall, resulting in cracking and consequent leakage between the tube and the tubesheet.
Various solutions have been suggested for addressing the problems associated with the crevice between the tubesheet and the tubes. U.S. Pat. No. 4,221,263 describes a solution including welding the tubes to the back face of the tubesheet. This may be achieved by welding around the periphery of each tube from the exterior of the tube, typically requiring specially bent electrodes to reach the confined spaces around the closely packed tubes. Alternatively, the welding may be performed from within the tubes using a welding head to reach within the open end of the tubes to form circular welds which must completely penetrate the tube walls.
The difficulty of arc welding a tube to a tubesheet from within the tube is increased by asymmetry in the heat sinking ability along the length of the tube adjacent to the location where the tube extends past the back face of the tubesheet. In particular, the portion of the tube extending past the tubesheet has a much lower heat sinking capacity than the portion of the tube extending within the tubesheet such that it has proven difficult to conduct sufficient heat through the tube to the tubesheet to create a weld without the heat melting through and creating a hole in the tube. On the other hand, if a lower power is applied such that the tube is not melted away, the power may be insufficient to penetrate the tube to create a weld connection.